Surface light source systems of various types, useful for illuminating transmission type liquid crystal displays and so on, have been proposed and put to practical use. Of the surface light source systems practically used now, some systems, such as those of edge light type or of direct type, use emission sources that do not emit light two-dimensionally or that do not emit light in a sheet formation.
In a surface light source system of direct type, emission sources that emit light linearly are arranged in parallel with each other, for example. In this surface light source system, cold cathode fluorescent tubes and a transmission type display element, such as an LCD (Liquid Crystal Display) panel, are spaced, as needed. Between the cold cathode fluorescent tubes and the transmission type display element are placed several optical sheets, such as a diffusion sheet for diffusing light and a sheet for collecting light substantially in one direction.
However, such a surface light source system is sometimes insufficient in the property of collecting light emitted from the emission sources, although the surface light source system includes many optical sheets. Therefore, an LCD panel to be used in combination with the surface light source system is sometimes improved such that even light obliquely entering the LCD panel can keep the image quality good. However, even if the LCD panel is so improved, optical efficiency cannot be satisfactorily increased, and, moreover, the structure of the LCD panel is complicated, so that the cost for producing a liquid crystal display increases.
Further, of the surface of the LCD panel that faces the surface light source system of direct type, those portions that are close to the emission sources (cold cathode fluorescent tubes) and those portions that are apart from the emission sources (i.e., those portions that face the parts between two adjacent emission sources) are different in light intensity (luminance), so that the surface of the LCD panel tends to have non-uniformity of light intensity (non-uniformity of luminance). Furthermore, light emitting diodes that emit light as point like are sometimes used as emission sources in these days. In the examples disclosed in Japanese Laid-Open Patent Publication No. 115372/2005, a light source unit is made by arranging light emitting diodes of red, green and blue, the three primary colors of light, on a plane. Point light sources arranged on a plane produce two-dimensionally non-uniform luminance. When emission sources that are different in the wavelength range of light they emit are used, the non-uniformity of color is brought about in addition to the non-uniformity of luminance.
By making the distance between the emission sources and the LCD panel greater, the surface of the LCD panel can be prevented from becoming non-uniform in luminance and in color. A greater distance between the emission sources and the LCD panel, however, brings about another problem that the display has an increased thickness. Similarly, the surface of the LCD panel can also be prevented from becoming non-uniform in luminance and in color by improving the light-diffusing properties of each optical sheet, or limiting the quantity of light each optical sheet transmits. These techniques, however, cause the problem that the efficiency of utilization of the light emitted from the emission sources decreases. Further, the surface light source systems disclosed in Japanese Laid-Open Patent Publications No. 119703/1993 and No. 242219/1999 use light-shielding members (lighting curtains, light-shielding dot layers) in order to prevent the surface of an LCD panel from becoming non-uniform in luminance and in color. This technique also causes the problem that the efficiency of utilization of the light emitted from the emission sources decreases.
The above emission sources consisting of light emitting diodes are sometimes supported on a printed circuit board. For example, in the case where the emission sources consisting of light emitting diodes are supported directly on a printed circuit board as in the surface light source system disclosed in Japanese Laid-Open Patent Publication No. 18261/2006, light from the emission sources partly reaches the printed circuit board. Since the light that has reached the printed circuit board is absorbed by the printed circuit board, the efficiency of utilization of the light emitted from the emission sources decreases.
Further, the printed circuit board is made using a glass epoxy resin, for example. The glass epoxy resin, however, has low thermal conductivity and is poor in heat-radiating characteristics. The heat of the emission sources, therefore, is not radiated, and the temperature of the emission sources rises. If the temperature of the emission sources consisting of light emitting diodes increases, there occur the problems that the light the light emitting diodes emit undergoes change in color and that the life of the light emitting diodes decreases.